1. Field of the Invention
The present invention relates to a phosgene-free process for preparing organic monoisocyanates by thermally decomposing compounds containing biuret/urea groups and prepared in situ by the reaction of isocyanate compounds and primary monoamines.
2. Description of the Prior Art
The preparation of organic isocyanate compounds with phosgene is well known and described in numerous publications and patents (for example Houben-Weyl, Methoden der organischen Chemie [Organic Chemistry Methods], Vol. 8, p. 120 et seq. (Georg Thieme Verlag Stuttgart 1952)). Special techniques and costly production and safety systems are a prerequisite for the safe handling of phosgene. For this reason there has been no shortage of attempts to synthesize isocyanate compounds by phosgene-free methods.
One simple laboratory method which is known for the phosgene-free preparation of isocyanates is a thermal decomposition of compounds having biuret or urea structures. A low molecular weight amine or polyamine is incorporated into an excess of a high boiling isocyanate compound, and the low boiling isocyanate formed in an equilibrium reaction is removed by distillation at temperatures of 200xc2x0 C. and above. The publication Bunge, W., Angew. Chem. 72, 1002 (1960) gives details of this xe2x80x9cSimple laboratory method for the preparation of low boiling isocyanatesxe2x80x9d. The publication Siefken, Annalen der Chemie 562, 81 (1949) also outlines this method with temperatures greater than 200xc2x0 C. being described. Patent application EP-A 307 756 and the publication W. Mormann, G. Leukel, Synthesis 12, 990 et seq. (1988) optimize the principle of this method for special siloxyisocyanates.
For numerous reasons it is difficult to optimize the processes mentioned. For example, at temperatures of 200xc2x0 and above polyisocyanates react in a manner no longer controllable and sometimes with evolution of gas, to form high molecular weight secondary products, such that at these high temperatures unpredictable reaction processes may occur in an industrial process.
There is little possibility of removing the high molecular weight secondary products from the reaction vessel on an industrial scale because the viscosity is too high. For reasons of viscosity it is not possible industrially to react equimolar quantities of high boiling polyisocyanates and low boiling amines, such as described in EP-A 307 756.
An object of the present invention is to provide a phosgene-free process for the preparation of monoisocyanates, which has broad applicability and can be carried out effectively, especially on a large industrial scale.
This object may be achieved with the process according to the invention, which is described in greater detail hereinafter.
The present invention relates to a process for preparing a low boiling monoisocyanate having a boiling point of between 70 and 320xc2x0 C. at standard pressure by reacting
A) a high boiling isocyanate compound having a boiling point of at least 180xc2x0 C. at standard pressure and an HC content of at least 50 ppm with
B) a monoamine having a primary amino group,
at a maximum reaction temperature of 180xc2x0 C. and a molar ratio of isocyanate groups to amino groups of at least 4:1 to form a compound containing a biuretlurea group, simultaneously thermally decomposing this compound in situ to form a monoisocyanate corresponding to monoamine B) and removing the monoisocyanate by distillation, optionally under vacuum.
High boiling isocyanate compounds A) are compounds and mixtures having isocyanate groups and boiling points of above 180xc2x0 C., preferably above 250xc2x0 C. and more preferably above 300xc2x0 C. under standard conditions.
Under the reaction conditions the boiling temperature of isocyanate component A) must be at least 10xc2x0 C., preferably 20xc2x0 C. and more preferably 40xc2x0 C. above the adjusted reaction temperature.
Suitable isocyanate compounds A) are known and include compounds having aliphatically, cycloaliphatically, araliphatically or aromatically bound isocyanate groups. Examples include monoisocyanates such as stearyl isocyanate and naphthyl isocyanate; diisocyanates such as 1,4-diisocyanatobutane, 1,6-diisocyanatohexane (HDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 2,2,4- or 2,4,4-trimethyl-1,6-diisocyanatohexane, 1,10-diisocyanatodecane, 1,3- and 1,4-diisocyanatocyclohexane, 1,3- and 1,4-bis(isocyanatomethyl)cyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate, IPDI), 4,4xe2x80x2-diisocyanatodicyclohexyl methane, 1-isocyanato-1-methyl-4(3)-isocyanatomethyl cyclohexane (IMCI), bis(isocyanatomethyl)norbornane, 2,4- and/or 2,6-diisocyanatotoluene (TDI), 2,4xe2x80x2- and/or 4,4xe2x80x2-diisocyanatodiphenylmethane and higher homologs, 1,5-diisocyanatonapbthalene and dipropylene glycol diisocyanate; triisocyanates and/or higher functional isocyanates such as 4-isocyanatomethyl-1,8-octane diisocyanate (nonane truisocyanate), 1,6,11-undecane triisocyanate; and mixtures of these isocyanate compounds.
Modified isocyanate compounds prepared from the preceding diisocyanates and triisocyanates by oligomerization reactions, such as trimerization, are also usable. Mixtures of the modified and unmodified isocyanates may also be used.
Compounds containing aromatically bound isocyanate groups are preferably used. Polyisocyanates of the diphenylmethane series having a bicyclic content (total of 2,2-, 2,4- and 4,4-diphenylmethane diisocyanate) of at least 85 wt. %, based on the total weight of the isocyanate component A), are preferably used as the isocyanate component A).
It is essential that the isocyanate component A) has an HC content (hydrolyzable chlorine compounds content) of at least 50 ppm, preferably at least 150 ppm and more preferably at least 300 ppm. This can be ensured either by an existing sufficiently high chlorine content of the isocyanate component A) due to its method of preparation, or by the addition of compounds which contain hydrolyzable chlorine. Examples of such compounds are benzoyl chloride, terephthaloyl dichloride and isophthaloyl chloride. The hydrolyzable chlorine content of the isocyanate component A) may be determined by known methods.
Any aliphatic, cycloaliphatic or aromatic compounds having a primary amino group and where the monoisocyanates forming as reaction products can be removed by distillation from the reaction mixture under the reaction conditions, may be used as low molecular weight monoamines B). The monoamines may contain, in addition to the amino group, other functional groups that are inert to isocyanate groups under the reaction conditions. The monoamines may be directly used at the purity available industrially without special purification.
Examples of suitable monoamines include C3-C18-alkylamines such as the isomeric butylamines, pentylamines, hexylamines, heptylamines, octylamines, nonylamines, decylamines and dodecylamines; C3-C18-alkylene amines such as alkylamine; monoamines based on optionally unsaturated, long-chain fatty acids; C5-C18-cycloalkylamines such as cyclohexylamine; aromatic amines such as phenylamine, ortho- and parafluorophenylamine, ortho- and para-chlorophenylamine and naphthylamine; alkyl phenylamines; and alkyl phenylamines containing halogen atoms. The carbon chains of the amines may be contain oxygen and/or sulfur atoms in the form of ether or thioether groups.
Monoamines containing an aromatically bound amino group are preferably used. Anilines containing halogen are especially preferred.
Monoisocyanates C) prepared according to the invention are derived from monoamines B) and must be distillable under the specified reaction conditions. They have a boiling point of at least 70xc2x0 C., preferably at least 110xc2x0 C. and at most 320xc2x0 C., preferably at most 240xc2x0 C., at standard pressure. The molecular weight of these monoisocyanates is generally 83 to 270.
In the process according to the invention high boiling isocyanate component A) and monoamine component B) are reacted at a molar ratio of isocyanate groups to primary amino groups of at least 4:1, preferably 5:1 to 20:1 and more preferably from 6:1 to 8:1, and at a maximum temperature of 180xc2x0 C., preferably 80xc2x0 C. to 160xc2x0 C. and more preferably 120xc2x0 C. to 140xc2x0 C. Monoamine component B) may be incorporated in pure form or in a blend with other non-reactive compounds. The monoamine is preferably incorporated as a solution in a solvent that does not boil under the process conditions. The solution preferably has a concentration of 10 to 90%, more preferably 40 to 60%. Examples of suitable solvents include high boiling trialkyl phosphates or tritoluyl phosphates.
The reaction temperature and bottom temperature in the reaction vessel is limited to a maximum of 180xc2x0 C. Working is preferably at reaction temperatures of from 100xc2x0 C. to 170xc2x0 C. and particularly preferably from 120xc2x0 C. to 160xc2x0 C.
The removal by distillation of monoisocyanate C) may take place under ambient pressure or at reduced pressure, preferably at reduced pressure and more preferably at a pressure of 5 to 200 mbar.
The process according to the invention enables monoisocyanates to be industrially prepared simply and at yields of over 70%. The bottom product that forms may be handled without difficulty.
The purity of monoisocyanates C) is preferably over 90%, more preferably over 99%. Therefore, monoisocyanate C) may be used directly in modification reactions and as intermediates without further purification.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.